1. Field of the Invention
The present invention relates to a method of fabricating a lower substrate of a Plasma Display Panel (PDP) and particularly, to a method of fabricating a lower substrate of a plasma display panel capable of easily forming a separating wall of a high aspect ratio and preventing formation of air layer between a green sheet and substrate and generation of cracks between separating walls.
2. Description of the Background Art
Generally, a PDP is a flat panel display device for displaying images such as letters or graphics by emitting a fluorescent substance by 147 nm of ultraviolet ray generated in discharging He+Xe or Ne+Xe gas. Such PDP can be easily made as a thin film and large screen and accordingly, recently, technology for improving the picture quality is rapidly developed.
FIG. 1 is a perspective view showing a plane discharging type PDP in the conventional alternate current driving mode. As shown in the drawing, the PDP includes a lower glass substrate 14 having an address electrode 2 and an upper glass substrate 16 having a couple of electrodes 4. A separating wall 8 for separating a dielectric layer and discharging cell is formed on the lower substrate 14 and fluorescent substance 6 for generating a visible ray by being emitted by ultraviolet ray generated in plasma discharging is coated on the surface of the dielectric layer 18 and separating wall 9.
The dielectric layer 12 and passivation layer 10 are formed in order on the upper glass substrate. The dielectric layer 12 stores wall charge in plasma discharging and the passivation layer 10 protects the couple of electrodes 4 and dielectric layer 12 against sputtering of gas in plasma discharging and increase emitting efficiency of a secondary battery. Mixed gas of He+Xe or Ne+Xe is injected and sealed to each discharging cell.
The separating wall 8 for preventing electric and optical crosstalk among discharging cells is the most important factor for determining displaying quality and emitting efficiency of the PDP and accordingly, as the panel of the PDP becomes larger and highly finer, much study about the separating wall is performed. Conventionally, there are several applied methods for fabricating the separating wall, such as screen printing method, sand blasting method, additive method, photo-sensitive paste method, Low Temperature Cofired Ceramic on Metal (LTCCM) method and the like.
The screen printing method has an advantage that the process is simple and the cost is low. However, the screen and glass substrate 14 must be arrayed at every printing time and printing and drying of a glass paste must be repeated several times. Also, in case the screen and the glass substrate is wrongly arrayed, since the separating wall transforms, precision of the separating wall is lowered.
The sand blasting method has an advantage that the separating wall can be formed on a large substrate. However, since much amount of glass paste is removed by grinder (namely, grains of sand) in the sand blasting method, material is wasted, thus to increase the fabrication cost. Moreover, the method has a disadvantage that the glass substrate 14 can be cracked or damaged by the impact occurred by the grinder.
The additive method is also appropriate to form a separating wall on the large substrate, but there occurs a problem that the separating wall is broken (damaged) when the residual substance is generated or the separating wall is generated since the photo-resist and the glass paste are not easily separated.
In the photo-sensitive paste method, the used photo-sensitive paste costs much and it is difficult to expose the lower portion of the photo-sensitive paste.
Compared with the above described methods, since the LTCCM method is simple and fabrication of the separating wall with high precision and high ratio, recently, the method is most widely used.
FIGS. 2A to 2G are views showing a lower substrate of the conventional plasma display panel using the Low Temperature Cofired Ceramic on Metal (LTCCM) method. Firstly, as shown in FIG. 2A, the green sheet 30 is fabricated. The green sheet is fabricated by positioning a slurry containing glass powder, organic solution, plasticizer, bond, additive and the like at a predetermined rate on a polyester film, forming the slurry in the shape of a sheet by a doctor blading process and then drying the resultant material.
As shown in FIG. 2B, the green sheet 30 is laminated-combined with the substrate 32. The substrate 32 is composed of glass, glass-ceramic, ceramic, metal and the like. Here, as metal used as material of the substrate 32, titanium is mainly used. Since titanium has higher strength than the substrate made of glass or ceramic material and higher heat-resistant temperature, with titanium, the substrate can be fabricated thinner than the substrate made of another substance such as glass or ceramic material and mechanical transformation can be minimized. Also, since titanium has high reflectibility, emitting efficiency and brightness can be increased by reflecting a visible ray back scattered to the side of the displaying surface.
In case the material of the substrate 32 is metal, it is desirable that fine glass powder is injected on the substrate 32 in the dry process or wet process before combining the substrate 32 and green sheet 30 so that the combination between the metal surface and green sheet 30 is easy. The injected fine powder is heated at the temperature of about 500 to 600° C. and fused and attached. The green sheet 30 is combined by laminating on the fused and attached metal substrate 32 on which the glass powder is fused and attached.
Then, as shown in FIG. 2C, the address electrode 2 is printed on the green sheet 30 and is dried.
As shown in FIG. 2D, the electrode passivation layer 36 is formed by drying the dielectric slurry after printing the slurry. Then, the substrate is heated, to under the softing point of organic material used as additive, for instance, polyvinylbutiral (PVB) to improve fluidity of the green sheet 30 combined to the substrate 32, after performing secondary laminating process to improve adhesive force between the green sheet 30 and electrode passivation layer 36.
Under the condition that the fluidity of the green sheet 30 is increased, after arraying metallic pattern 38 where a groove 38a is formed as shown in FIG. 2E, the substrate 32 is pressurized in the metallic pattern 38 with the pressure higher than about 150 kgf/cm2 as shown in FIG. 2F. By such pressurization, the green sheet 30 and electrode passivation layer 36 move into the groove 38a of the metallic pattern and rise up, thus to form a separating wall.
Then, as shown in FIG. 2G, the separating wall is plasticized by the heating, maintaining and cooling processes of the green sheet 30 and electrode passivation layer 36 after separating the metallic pattern 38 from the substrate 32. In the plasticizing process, organic material in the green sheet 30 is burned out by heat and crystalline nuclear is generated and grown up in inorganic material at a higher temperature than the burn-out temperature.
After plasticizing the separating wall, reflecting material such as TiO2 is printed on the electrode passivation layer 36 and plasticized before printing the fluorescent substance 6.
As described above, with the LTCCM method, the process can be simple and separating wall can be formed in high precision. However, in the LTCCM method, formation of the separating wall 8 in the high aspect rate having larger height than the width is difficult and the green sheet 30 protruded in the shape of the separating wall is torn in separating the metallic pattern 38 and green sheet 30 or an air layer is generated between the substrate 32 and the green sheet 30 in forming by pressurizing. Such problem is caused by organic material contained in the green sheet 30. In case the amount of organic material in the green sheet 30 is large, the fluidity of the green sheet 30 is improved, but the height of the shaped separating wall is lowered again when the organic material is burnt out in plasticizing the green sheet 30 and the electrode passivation layer 36 after moving the organic material having higher fluidity into the groove 38a of the metallic pattern in forming the separating wall. Also, the portion protruded into the shaped separating wall 8 (upper portion of the separating wall) is torn in separating the metallic pattern 38 and green sheet 30.
On the other hand, since the fluidity of the green sheet 30 is low in case the amount of the contained organic material in the green sheet 30 is small, movement of the green sheet 30 into the groove of the metallic pattern 38a is difficult and accordingly, the separating wall can not be formed.
Also, with the conventional method of fabricating the separating wall using the LTCCM method, the air layer 40 is generated between the green sheet 32 and substrate 30 by difference of frictional force in shaping the wall as shown in FIG. 3. Such air layer 40 lowers strength of the separating wall 8 and causes leakage of gas. The difference of interfacial frictional force between the green sheet 32 and substrate 30 causes generation of cracks 42 among the separating walls as shown in FIG. 4 since the adjacent separating walls 8 move in the different direction.